Make Jamulus iOS compatible with Qt6

**Describe the bug**

Even though Jamulus can be compiled with Qt6 and it runs, there are multiple UI issues.
Also the app hangs and crashes in multiple places.

1. The chat window doesn't have the correct proportions. Especially if turning the device around the chat window overflows the screen
2. Tapping the chat window Close button hangs and then crashes the app on the next tap.
3. Changing the language in settings doesn't apply and the app hangs.
4. Further bugs to be investigated

Related: https://github.com/jamulussoftware/jamulus/issues/2711

1.

3. Probably the app cannot display the message saying that the app needs a restart: If I remove QMessageBox::information(), the app doesn't hang and after a restart the language is changed correctly.

**Operating system**

iOS 17

**Version of Jamulus**

Latest version from main

**Additional context**

Try compiling Jamulus with Qt6 for iOS.

Start by compiling the latest main branch with Qt6 for iOS and reproducing the reported chat-window and language-change failures on iOS 17. Done means the chat layout remains usable in portrait and landscape, closing the chat no longer hangs or crashes, and changing the language works without the reported hang.

Written by the indexing model from the issue text.